1. Field of the Invention
The present invention relates to an active matrix substrate, a liquid crystal display (LCD) device, and a method of manufacturing the LCD device. Particularly, the present invention relates to an active matrix substrate which forms an LCD device of a horizontal electric field type, the LCD device of the horizontal electric field type, and a method of manufacturing the LCD device of the horizontal electric field type.
2. Description of the Related Art
Recently, an active matrix LCD device using a thin film transistor (hereinafter referred to as a “TFT”) as a switching element of a pixel has been widely used as a high-resolution display. The active matrix LCD device is configured mainly of a TFT substrate having an array of the TFTs that act as the switching elements, a counter substrate opposed to the TFT substrate, and a liquid crystal layer sandwiched between the substrates. This LCD device is classified into a vertical electric field type and a horizontal electric field type. The vertical electric field type is designed to rotationally change the direction of directors of oriented liquid crystal molecules in a direction perpendicular to the TFT substrate. The horizontal electric field type is designed to rotationally change the direction of the directors of the oriented liquid crystal molecules in a direction parallel to the TFT substrate. The vertical electric field types include, for example, a twisted nematic (TN) type, and the horizontal electric field types include, for example, an in plane switching (IPS) type.
Each pixel of the LCD device of the horizontal electric field type is provided with a pixel electrode and a common electrode, which are formed on the TFT substrate in parallel and alternate relation, to apply a voltage across the electrodes and thereby generate an electric field parallel to the plane of the substrate. This type of display is designed to apply the electric field parallel to the plane of the substrate, thereby change the direction of the directors of the liquid crystal molecules, and thereby control the amount of transmitted light. The vertical electric field type has a problem that the relation between the amount of transmitted light and the applied voltage, as viewed from the direction of the directors, varies greatly from that as observed from the normal direction to the substrate. In contrast, the horizontal electric field type does not present a problem as is the case with the vertical electric field type because the directors of the liquid crystal molecules rotate in the plane of the substrate. The LCD device of the horizontal electric field type is therefore capable of achieving an image of good quality, as viewed from a very wide viewing angle.
Within each of the pixels of the LCD device of the horizontal electric field type, the liquid crystal layer is driven by being subjected to the electric field originating from the pixel electrode and the common electrode. A signal line receives feed of data written through the switching element to the pixel electrode of each pixel. Alignment defect of the liquid crystal molecules may develop at the edge of the pixel, particularly in the vicinity of the signal line, due to the action of the electric field leaking from the signal line upon the liquid crystal molecules. This involves the occurrence of leakage of light and the like, thus leading to degradation in display quality. The approach of providing a black matrix for light shielding is adopted in order to prevent the degradation in the display quality. Specifically, the black matrix is disposed in a region, facing the edge of the pixel, of the counter substrate opposed to the TFT substrate.
When the width of the black matrix is expanded in a case where the electric field leaking from the signal line has a great influence, the area of a region available for display (namely, an aperture of the pixel), however, decreases correspondingly. Proposed are structures adopted to suppress the electric field leaking from the signal line without having to narrow the aperture of the pixel. However, neither of the structures are satisfactory.